1. Field of the Invention
This invention relates to a method and apparatus for reducing speckle effects when a laser beam is reflected from a surface, and more particularly to the use of a chirped speckle-reducing laser beam that is compatible with nonlinear optical processes.
2. Description of the Related Art
When laser light is reflected from an extended three-dimensional object, a sparkling or speckled pattern referred to as laser speckle is typically observed in the reflected light. The speckle pattern results from wave interference. Each point on the surface of the object scatters the illuminating laser light as a spherical wave, so that the overall surface can be approximated as a collection of a great number of closely packed point sources. Because of the three-dimensional (i.e. non-planar) shape of the surface, the relative phases of the point sources depend in a systematic manner on the locations of the point sources. When the reflected beam is focused onto a detector or imaging device, wave contributions from a number of different points on the illuminated surface are present at any given point on the focal plane, each with its distinct phase. The superposition of these waves produces an irregular interference pattern, known as the speckle pattern.
A growing number of advanced electro-optical systems employ some form of active imaging or tracking, such as using a laser to illuminate a distant target that is then imaged and tracked. A common problem with such laser illumination is that the resulting images are often degraded due to speckle effects.
The interference which gives rise to laser speckle is a manifestation of laser coherence. One approach to eliminating or at least reducing speckle effects has been to specify that the laser source have a relatively short coherence length (for example, on the order of 1 cm). This technique of minimizing interference effects by utilizing a short coherence length is described, for example, in Born and Wolf, Principles of Optics: Electromagnetic Theory of Propagation, Interference and Diffraction of Light, 5th ed., Pergamon Press, 1975, pages 316-323. In principle, a short coherence length is easily achieved by operating the laser oscillator in many longitudinal modes simultaneously. In practice, however, while such multi-mode operation is indeed effective in reducing speckle effects, it complicates laser source development because it is incompatible with several common nonlinear optical processes, and can preclude the use or reduce the effectiveness of such processes. Examples of such nonlinear optical processes are stimulated Brillouin scattering (SBS) phase conjugation used for beam cleanup, harmonic generation for wavelength conversion, and Raman applications for beam combining or wavelength conversion. There is a basic incompatibility between the long coherence length that is necessary for effective SBS phase conjugation or other nonlinear optical processes, and the short coherence length used for speckle reduction.
Another approach to eliminating speckle effects is to employ stimulated rotational Raman scattering (SRRS) to generate multiple wavelengths from a single initial wavelength. This technique is described in U.S. patent application Ser. No. 08/004,166, "Speckle Suppression Illuminator" filed Jan. 11, 1993 by Rafanelli et al. and assigned to Hughes Aircraft Company, the assignee of the present invention. In this approach the initial laser beam can possess a high degree of coherence. However, there are several disadvantages, including a relatively high degree of complexity, less than optimum output beam quality, optical design problems arising from chromatic effects, limited power scalability and relatively ineffective speckle reduction.
In the Rafanelli et al. application the output of the prime laser, which is assumed to be a frequency-doubled Nd laser producing a wavelength of about 530 nm, is directed into a low-pressure hydrogen gas cell in which multiple-order rotational Raman shifts are imparted to the beam by stimulated rotational Raman scattering (SRRS). Each shift produces a new wavelength that is separated from the lower Stokes order by about 10-20 nm. Present indications are that 8-10 lines might be produced, with a total energy amounting to about 80% of the initial input energy.
In addition to considerable complexity, SRRS converters typically produce an increasingly poor beam quality in the higher Stokes orders; this reduces the effectiveness of the speckle reduction technique in most anticipated applications, which generally require a high beam quality. There are also serious challenges in designing optics and coatings that can accommodate the multiple SRRS beams (which typically span a total wavelength range in excess of 100 nm), because of chromatic effects in coatings and optical materials. It has furthermore been shown that speckle reduction with the SRRS approach scales roughly as the square root of the number of Stokes orders; this weak dependence arises from the wide wavelength separation of the Stokes lines.